Co-ingestion of protein with carbohydrate during recovery from endurance exercise stimulates skeletal muscle protein synthesis in humans.
Co-ingestion of protein with carbohydrate (CHO) during recovery from exercise can affect muscle glycogen synthesis, particularly if CHO intake is suboptimal. Another potential benefit of protein feeding is an increased synthesis rate of muscle proteins, as well documented after resistance exercise. In contrast, the effect of nutrient manipulation on muscle protein kinetics after aerobic exercise remains largely unexplored. We tested the hypothesis that ingesting protein with CHO after a standardized 2 h bout of cycle exercise would increase mixed muscle fractional synthetic rate (FSR) and whole body net protein balance (WBNB) versus trials matched for total CHO or total energy intake. We also examined whether post-exercise glycogen synthesis could be enhanced by adding protein or additional CHO to a feeding protocol that provided 1.2 g CHO(.)kg(-1.)h(-1), which is the rate generally recommended to maximize this process. Six active men ingested drinks during the first three h of recovery that provided either 1.2 g CHO(.)kg(-1)(.)h(-1) (L-CHO), 1.2 g CHO +0.4 g PRO(.)kg(-1)(.)h(-1) (PRO-CHO) or 1.6 g CHO(.)kg(-1.)h(-1) (H-CHO) in random order. Based on a primed constant infusion of L-[ring-(2)H5]-phenylalanine, analysis of biopsies (v. lateralis) obtained at 0 and 4 h of recovery showed that muscle FSR was higher (p<0.05) in PRO-CHO (0.09+/-0.01 %(.)h(-1)) versus both L-CHO (0.07+/-0.01 %(.)h(-1)) and H-CHO (0.06+/-0.01 %(.)h(-1)). WBNB assessed using 1-(13)C-leucine, was positive only during PRO-CHO and this was mainly attributable to a reduced rate of protein breakdown. Glycogen synthesis rate was not different between trials. We conclude that ingesting protein with CHO during recovery from aerobic exercise increased muscle FSR and improved WBNB, as compared to feeding strategies that provided CHO only matched for total CHO or total energy intake. However, adding protein or additional CHO to a feeding strategy that provided 1.2 g CHO(.)kg(-1)(.)h(-1) did not further enhance glycogen resynthesis during recovery. Key words: protein turnover, stable isotopes, amino acids, glycogen.